World Core is a gigantic Planet Kaioh. On World Core, a gigantic tree on the planet known as the World Tree spawns Shinjin from its fruit. The Shinjin go to a castle that's similar to a school and learn a variety of things. The population of the planet is about 80.
The World Tree (界樹 Kaiju?) is a gigantic tree found on World Core that grows fruits which is able to bring life to the Shinjin. Shinjin are selected to become a Kaioh or a Kaiohshin. However, a Kaiohshin is only selected if you are born from a golden fruit, which is rare. On occasion, Shinjin with evil hearts are born and fall under the Makaiohshin.
The School go to a castle that is similar to a school to learn a variety of information.